How do you measure background or RTC?

• A. Set emissivity to 0.5, set distance to 10, measure glass with the average tool
• B. Set emissivity to 1.0, distance to 0, measure crumpled aluminum foil with the average tool
• C. Set emissivity to 0, distance to 5, measure water surface
• D. Use the camera's automatic background mode

Answer: (B)

Measuring background or RTC relies on using a reference target that acts as a stable, near-ideal emitter so the camera can calibrate its readings against a known radiance. The best way to do this is to present a surface that behaves like a near-blackbody, is diffuse (not shiny), and is observed at a very short distance so atmospheric effects and reflections from the surroundings are minimized. Setting the emissivity to 1.0 treats the target as a perfect emitter, and placing it at a distance of zero ensures you’re measuring the target’s own emission directly rather than a mix of emissions and reflections. A crumpled aluminum foil works well here because crumpling creates a rough, diffuse surface that reduces specular reflections and yields a more uniform emissivity, making the target a reliable reference. Using the camera’s average tool helps stabilize the reading for a repeatable RTC reference.

The other approaches introduce complications: measuring through glass at a mid-range distance can cause reflections and transmission effects that distort the reference; setting emissivity to 0 misrepresents the target’s emission since many materials, including water, emit strongly in the infrared; and relying on an automatic background mode may not provide the controlled, repeatable reference required for accurate RTC calibration.